Embodiments of the present invention relate generally to gas analysis and more particularly to the measurement of gas density to determine gas flux.
When gas density measurements are performed by scanning a single rotational line or a few-discrete lines of analyte with a single-mode tunable laser source, the measured signal is temperature and pressure dependent due to a combination of Boltzmann population distribution of rotational levels, Doppler broadening, and temperature-dependent pressure broadening of an individual line. These pressure and temperature effects could also be affected by the presence of water and other gases in the sampled air. The combination of all of these effects will be further referred to as T-P effects. In addition, if a constant mixing ratio gas is used at relatively constant pressure, measured gas density itself changes with temperature and water content due to thermal expansion and water dilution of the gas per the Ideal Gas Law.
When the temperature or water content of gas changes, T-P effects may lead to a large change in absorption, significantly affecting the gas density measurement. In general, the T-P effects are specific and different for each absorption line of each gas.
With slow measurements of gas density, taking seconds and longer, the T-P effects can be calibrated out because mean temperature and water content of gas in the sampling volume can be easily measured. With fast measurements of gas density, e.g., several or more times per second, it is difficult to correct for T-P effects on-the-fly because it would require accurate and precise measurements of gas temperature and water content integrated over the entire sampling cell volume, and recorded at the same exact moment when the absorption is measured.
Existing gas analyzers, especially for trace gases such as methane, nitrous oxide, isotopes of carbon dioxide and water, etc., are closed-path sensors requiring long intake tubes and powerful pumps to allow sample gas flow of 30-100 lpm (liters per minute) and more. The fast temperature changes are attenuated in these long intake tubes so slow temperature measurements can be used, but power consumption of such sensors systems goes up to 1000 Watts and more, making them difficult to use in remote locations where most of the natural gas exchange processes for these gases occurs.